Stabilization of sulfur trioxide



Patented Jan. 11, 1949 UNITED STATES GFFICE 23585718" I sreenllze'riou Fevseememme Harold GJMOCaIin, Flushing", assignor to Allied Chemical &Dye Corporation, a'corpo' ration of New York No Drawing.Applicationliehruary 9, 1946, Serial NO. 6465 699 Claims. 1 v

This invention relates to the stabilization of liquid sulfur tri'oxide(substantially pure S03) and high strength oleums.

At room temperature, liquid sulfur trioxide polymerizes to variousdegrees. The three generally recognized S03 modifications melt atapproximately 17 C., 325 C., and 62 0., respectively. On standing attemperature below about 27 C., in the presence of even small amounts ofmoisture, the form melting at about 17 C. rapidly polymerizes to thehigher melting forms, the mass soon becomes solid, and melts only underpressure, sometimes requiring temperatures up to 100 C. to eifectcomplete liquefaction. Oleums of free 803 strengths upward of about 80%tend to polymerize in similar fashion, degree/0f polymerizationdepending chiefly upon the S02 strength of the oleum and teniperaturesatwhich the material is stored. In oleums of the lower portions of the S0strength range indicated,

certain polymers form, the degree of polymerization increases on passageof time,,and while the material may not solidify completely, theultimate mass may be a liquid phase containing in suspension the varioussolid polymers noted;

In many industrial processes, it is highly desirable to use S03 initsstrongest obtainable form However, the foregoing characteristics ofsulfur trioxide and high stren'g'tholeums create marked disadvantagesrelative to the storage, shipment and use of such materials. While, forexample, sulfur trioxide might be shipped in drums equipped with heatingcoils, there'is' involved the first problem of supplying a relativelyexpensive form of container. Moreover, since heat transfer thru a solidmaterial is poor, the solidified .sulfur trioxide in the container canbe melted by theuser only with considerable difficulty. In addition tothis disadvantage; after relatively long periods of standing, a largeproportion of the sulfur trioxide will have polymerized to the highmelting form in which case complete melting can only be eifected byheating at high temperature under considerable pressure.

To a lesser extent the same disadvantages apply to high strength oleums.

This invention aims to provide methods for making stabilized forms ofliquid sulfur tr'i'ox'ide or high strength oleums which either do notpolymerize to any substantial degree and remain liquid at roomtemperature, or in which, if polymerization does take place, thepolymers formed are readily meltable at moderate temperatures of about40 C. and below, i. e. temperatures which do not require'substantialvaporization of $03, or

2 use of heating containers capable of withstanding ely high pressures,as is the case whenhigh v tures are needed to melt the more're'refractory types of polymers.

,Ac'cording'to' this invention, it hasbeen found that the incorporationof relatively srn'all amounts of boron compound, with substantially 100%sulfur trioxide or oleums of S03 strength such that S0 polymers tend to"form, ac'comfpl'ishes' the advantageous objectives 'stat e'd bo've,

Specific modes of :arrying out theinventi'o'n depend u'pon" whether itis' desired to m'aEe a finished" product which does not polymerize toany substantial extent" remains" liquid" at about room temperature,orwh'ether the desired product is one whiclima'y contain solid polymerswhich are easily meltable at temperature of about C i le -J v Asindicated above, oieumsof SO5 strength of aboiit upward 'te n'd topolymerize" to greater or less extent dependihginostlyupoir'spe strengthandtime and temperature of storage. It has been found that thefmere'incorporationjof a boron compound, with oleums of S03 strength ofabout v80 and upward or wi thliqu'id substafi tial'ly'100% sulfurtrioxide", results in a product in which, if it contains" polymers, suchpolymers are ea s'ily 'rnel table at temperature'of ahout 40 C. or less.H i} Suitable boron compoundswhich may 'be us'e'd in carrying out theinvention arethose" of the class consisting of the oxides; and halidesof boron and the inorganic derivatives thereof such asboron momeaor;borontrichloride, BCla lboron triiiuoride, BFs; boraX, NazBiOv; sodiumand; potassium fiuoborates, NaBFi and KBF4; boric acid itself, H2303,orlany of its dehydrated forms such as metabolic acid, HBOz'or pyroboricacid, P121340 The preferred treating material is boron tri'oxid'e,andthe next most desirable material: is boron trichlo'ride. I

Choice of the'particular treating material to be employed dependsu'p'ofn whether it i is desired to make a product which containssubstantially nopolymers and remains liquid at 'about rooin temperature,or to'make'a' product whichrriay contain polymers hutwhich polymers are'inel'table attemperature belowabout 40 0.; whether it is objectionableto introduce; water int'd the product {which water may'b'e broughtiniwithjhe treating material either as free water or'water ofconstitutionigor whether according tothe "ch:- cufrisfianees'it'isundesirable toformja final productwhichniay contain a small ari ountof'precipitater When itis desired to make a product which may containpolymers, but which are meltable at temperatures less than about 40 0.,and it is immaterial whether or not some water is brought into theproduct along with the treating agent, and where it is immaterialwhether the product contains a small amount of precipitate, any of the.indicated stabilizing agents may be used. If the sought-for productshould not contain a precipitate, Na2B4O7, KBF, NaBF4 should not beemployed. On the other hand, if circumstances be such that it is notdesirable to introduce water in any form into the product, the treatingmaterial should contain substantially no free water nor water ofconstitution, free water being used in the sense as to include occludedmoisture and water of crystallization.

' The quantities of treating agent employed depend chiefly upon thedegree of polymerization inhibition desired. It has been found that noadditional advantages are obtained when the treating material is used inamount equivalent to more than 1.5 by weight of boron. Ordinarily it ispreferred to utilize treating material in amount equivalent to not morethan 0.6% by weight of boron, the latter quantity being generallyappreciably more than adequate to effect stabilization of sulfurtrioxide or oleum of S03 strength of about 80% and up when thesought-for product is such that if the product contains polymers suchpolymers are readily meltable at temperature below about 40 C.

In accordance with another phase of the invention, it has been foundpossible to make high S03 strength products which do not polymerize toany appreciable extent and which remain substantially in liquid formwhen brought to room temperature even though previously stored forseveral weeks at temperature of zero degrees C. and less. Thisembodiment of the invention comprises treating substantially 100% liquidsulfur trioxide or oleum of S03 strength not less than 99.5% (balanceH2SO4) with a boron compound containing substantially no free water norwater of constitution, and then subjecting the resulting material to aheat treatment. When practicing this aspect of the invention and it isdesired to make a product which does not contain a precipitate, suitablestabilizing agents to employ are B203, B013 or BF3, anhydrous B203 beingthe preferred material. On the other hand, if a small amount ofprecipitate in the product is unobjectionable, KBF4, NaBFq. or Na3B4Ovmay be used. However, in the best embodiments of the invention, use ofthese latter agents is not preferred since in some circumstances if theresulting treated materials are stored at temperatures below theirfreezing points, there may form some polymers which require heating toabout 40 C. to effect melting.

In the practice of the preferred embodiments, the quantities of treatingmaterial incorporated may be about the same as previously indicated,although in this situation it is rarely necessary to use the boroncompound in amount equivalent to as much as 0.6 by weight, and often,particularly when using B203, the quantity of the latter may be aslittle as equivalent to 0.03% by weight of boron.

After incorporation of the treating material, the heat treatment may becarried out in a closed Vessel, suitable to withstand any pressuregenerated, at temperature ordinarily not less than about 60 C. andusually in the range of 60 C. to 100 0. Duration of the time of heattreatment is dependent largely upon the degree of polymerizationinhibition desired. Any appreciable amount of heat treatment even at thelow temperature of about 60 C. markedly cuts down the degree ofpolymerization, and if a product which is largely liquid phase butcontains some solid phase at about room temperature is unobjectionable,heating at the low temperature of about 60 C. for about an hour or somay be employed. The greater the desired state of liquidity of theproduct at temperature of about 20 C., the greater the temperature andtime of heating of the heat treatment stage should be. Preferably, heattreatment is carried out for a period of from 2 to 10 hours Thepreferred products of the invention, 1. e. those which do not polymerizeto any appreciable extent and which remain substantially in liquid format about room temperature, preferably should be kept sealed off fromcontact with air since after heat treatment a small amount of moisturecauses such products to lose some stability and to form polymers whichmay require heating to about 40 C. to effect melting.

The sulfur trioxide or oleum to be treated may be obtained from anysuitable source. For example, substantially S03 may be obtained bydistilling say 30% oleum under conditions to avoid vaporization ofH2S04, and condensing the S03 at temperature, say 20-30 C., at whichliquid sulfur trioxide has suificiently low vapor pressure. Similarly,liquid sulfur trioxide may be obtained from the dry mist free, say 10%S03 gas exiting a catalytic S02 oxidation operation, by compressing thegas to about -470 lbs. per square inch, and cooling the compressed .gasto 2025 0., under which conditions about 70% of the S03 may be liquefiedand recovered as substantially 100 liquid sulfur trioxide.

The treating material used may be added directly to the liquid to bestabilized, or in the case of stabilizin substantially 100% sulfurtrioxide, the vaporous S03 conveniently may be condensed in a closedvessel in which the treating material to be used has been previouslyplaced.

According to one example of practice of the invention, oleum of about60% free S03 strength was distilled under conditions to avoidvaporization of H2S04, the S03 vapor was condensed in a I closed vesselat temperature of about 20 0., anhydrous B203 stabilizing agent havingbeen previously placed in the condenser. The amount of anhydrous B203used was equivalent to 0.09% of boron based on the total weight ofcondensed S03. After all of the S03 used in the run was condensed, theliquor mass in the condenser was agitated, and there was obtained aproduct which, on standing at temperatures less than 20 0., containedsome polymers, but all of such polymers melted When the treated materialwas heated to about 40 C.

In another example, S03 was distilled out of oleum containing 60% S03under conditions to prevent distillation of any E2804, and the S03 vaporwas condensed at temperature of 25 C. in a closed container into whichanhydrous B203 had been previously introduced. The quantity of 13203used was equivalent to 0.03% of boron based on the Weight of the totalamount of liquid S03 treated. When all the S03 to be stabilized had beencollected in the condensing Vessel, the latter was tightly sealed andheated at temperature of about 80 C. for about 8 hours with occasionalagitation, the liquid was allowed to cool at room temperature, and theresulting product was such as to be liquid at room temperature evenafter having been stored for several weeks at temperature of about C.

I claim:

1. The method of stabilizing material of the group consisting of sulfurtrioxide and oleum of S03 strength such that S03 polymers tend to form,which method comprises incorporating with said material a boron compoundin amount equivalent to not more than 1.5% by weight of boron.

2. The method of stabilizing material of the group consisting of sulfurtrioxide and oleum of S03 strength such that S03 polymers tend to form,which method comprises incorporating with said material a compound ofthe class consisting of the oxides and halides of boron and theinorganic derivatives thereof, in amount equivalent to not more than1.5% by weight of boron.

3. The method of stabilizing material of the group consisting of sulfurtrioxide and oleum of S03 strength not less than 80%, which methodcomprises incorporating with said material a boron compound in amountequivalent to not more than 1.5% by weight of boron.

4. The method of stabilizing material of the group consisting of sulfurtrioxide and oleum of S03 strength not less than 80% which methodcomprises incorporating with said material a compound of the classconsisting of the oxides and halides of boron and the inorganicderivatives thereof, in amount equivalent to not more than 0.6% byweight of boron.

5. The method of stabilizing material of the group consisting of sulfurtrioxide and oleum of S03 strength not less than 80%, which methodcomprises incorporating boron trioxide with said material in amountequivalent to not more than 0.6% by Weight of boron.

6. The method of stabilizing material of the group consisting of sulfurtrioxide and oleum of S03 strength not less than 99.5%, which methodcomprises incorporating substantially anhydrous boron trioxide with saidmaterial in amount equivalent to not more than 1.5% by weight of boron.

7. The method of stabilizing material of the group consisting of sulfurtrioxide and oleum of S03 strength not less than 99.5%, which methodcomprises incorporating with said material a compound of the classconsisting of the oxides and halides of boron and the inorganicderivatives thereof, in amount equivalent to not more than 1.5% byweight of boron, said compound containing substantially no free waternor water of constitution, and heating the resulting material attemperatures not less than 60 C. in a closed container for not less thantwo hours.

8. The method of stabilizing material of the group consisting of sulfurtrioxide and oleum of S03 strength not less than 99.5%, which methodcomprises incorporating substantially anhydrous boron trioxide with saidmaterial in amount equivalent to not more than 1.5% by Weight of boronand subjecting the resulting material to heat treatment.

9. The method of stabilizing material of the group consisting of sulfurtrioxide and oleum of S03 strength not less than 99.5%, which methodcomprises incorporating substantially anhydrous boron trioxide with saidmaterial, in amount equivalent to not more than 0.6% by weight of boron,and heating the resulting material at temperature of 60-100 C. in aclosed container for a period of from two to ten hours.

10. A composition comprising material of the group consisting of sulfurtrioxide and oleum of S03 strength such that S03 polymers tend toform,,and containing boron compound in amount equivalent to not morethan 1.5% by weight of boron.

11. A composition comprising material of the group consisting of sulfurtrioxide and oleum of S03 strength not less than and containing boroncompound in amount equivalent to not morethan 1.5% by Weight of boron,said boron compound having been derived from an incorporated borontrioxide.

12. A composition comprising material of the group consisting of sulfurtrioxide and oleum of S03 strength not less than 80%, and containingboron compound in amount equivalent to not more than 0.6% by weight ofboron, said boron compound having been derived from an incorporatedcompound of the class consisting of the oxides and halides of boron andthe inorganic derivatives thereof.

13. A composition comprising material of the group consisting of sulfurtrioxide and oleum of S03 strength not less than 99.5%, and containingboron compound in amount equivalent to not more than 0.6% by weight ofboron, said boron compound having been derived from incorporatedsubstantially anhydrous boron trioxide.

14. The method of stabilizing material of the group consisting of sulfurtrioxide and oleum of S03 strength not less than 99.5%, which methodcomprises incorporating with said material a compound of the classconsisting of the oxides and halides of boron and the inorganicderivatives thereof, in amount equivalent to not more than 1.5% byweight of boron, said compound containing substantially no free waternor water of constitution, and then subjecting the resulting material toheat treatment.

15. A composition comprising material of the group consisting of sulfurtrioxide and oleum of S03 strength such that S03 polymers tend to form,and containing boron compound in amount equivalent to not more than 1.5%by weight of boron, said boron compound having been derived from anincorporated compound of the class consisting of the oxides and halidesof boron and the inorganic derivatives thereof.

HAROLD G. McCANN.

REFERENCES CITED The following references are of record in the file ofthis patent:

Deutsche Chemische Gesellschaft (Berichte), 73B, pp. 1397-8.

Mellor: Inorg. and Theoretical Chem, vol. 5 (p. 146), Longmans, Green &00., N. Y.

